The present invention relates to a motor-driven pump having a brushless motor for pumping liquid.
A motor-driven pump having a pump section and a motor section for driving the pump section which are integrally connected is known in the prior art. Liquid is sucked from a liquid inlet of the pump into the pump section, and pressure of the liquid is increased in the pump section. The liquid having an increased pressure is fed through a passage defined between a rotor and a stator of the motor section to a liquid outlet of the pump. Although a brush motor having a commutator has been utilized for the motor section, it has a problem that a commutator tends to be worn. In the circumstances, a brushless motor having no commutator has been recently utilized for the motor section (Japanese Patent Laid-open Publication No. 56-88982 and 61-14496).
The brushless motor is classified into a sensor type having a position detecting element such as Hall device, magnetodiode and magnetoresistance device for detecting a pole position of the rotor and a sensorless type not having such a position detecting element. In the sensor type motor-driven pump, the pole position of the rotor formed of permanent magnet is detected by the position detecting element. As the result of detection, it is decided which of coils wound on pole portions of the stator should be excited. The excitation of the decided coil is switched on and off by a control circuit having a transistor and the like. The position detecting element is so provided as to sense a magnetic flux generated by a sensor magnet fixed to the rotor or a rotating shaft of the motor. Such a motor-driven pump is disclosed in Japanese Utility Model Laid-open Publication No. 62-59794, for example. The position detecting element is provided in the vicinity of the rotor so as to improve the sensitivity of the detecting element. Further, the control circuit is integrally connected to the pump section and the motor section so as to make the pump compact and easy to mount. Further, the control circuit is enclosed in a circuit package having a high heat conductivity from the viewpoint of protection of the control circuit from heat, and the circuit package is preferably located in a liquid passage in the pump so as to cool the circuit package. However, in the prior art motor-driven pump, a circuit board for mounting the position detecting element thereon is entirely fixed on an inner surface of a thin wall of the circuit package, and the thin wall tends to be exposed to repeated stress due to fluctuation in liquid pressure upon driving and stopping of the pump. As a result, there is a possibility of the circuit board being damaged by the pressure fluctuation to cause adverse affect on the detection accuracy. Although this problem can be eliminated by increasing a wall thickness of the circuit package, such an increase in wall thickness will cause a reduction in sensitivity of the position detecting element. Furthermore, it is necessary to ensure precise positioning between the position detecting element and the stator, so as to improve the detection accuracy and proper excitation timing for the coils.
On the other hand, in the sensorless type motor-driven pump, a rotor position is detected from a counter electromotive force generated by rotating the rotor. However, although the above-mentioned position detecting element is not needed, many special circuit such as dummy start signal generating circuit, normal start selector circuit and fail-safe circuit are required. As a result, the control circuit becomes complex, and the number of parts constituting the control circuit is increased to render the compactness of the pump difficult.